1. Field of the Invention
The present invention relates to refrigeration cooling systems and, more particularly, to a control unit for use in combination with refrigeration cooling systems for controlling the refrigeration compressor in response to pressure changes in the refrigerant flow in the system.
2. Description of the Prior Art
The majority of all commercial refrigeration or air conditioning systems operate upon the principal of compressing a refrigerant in a compressor, passing the compressed refrigerant through a condenser to remove heat from the compressed refrigerant and then passing the cooled and compressed refrigerant through an expansion valve and evaporator back to the compressor. In passing through the evaporator, the refrigerant picks up heat from the surrounding atmosphere so as to cool that atmosphere. Refrigerant flow is generally controlled by controlling operation of the compressor; in other words, by turning the compressor on and off depending upon the temperature of the area to be cooled.
To control actuation of the compressor, most larger refrigeration systems utilize a pressure actuated switch. The bellows or actuator of this switch is connected directly to the refrigerant line by a small capillary tube so that changes in pressure (and temperature which is directly proportional to pressure) result in actuation of the switch and consequently the compressor.
One of the shortcomings or weaknesses of a refrigeration system of the type discribed hereinabove is the frequency of failure of the system because of a failure of the capillary tube. Generally, this tube is made from 1/16 inch copper tubing connected at one end to the compressor and at the other end to the pressure switch control unit. Frequently, this tube breaks, either as a consequence of mechanical fatigue or as a consequence of a serviceman or other personnel accidentally bumping it. Breakage of this tube involves not only replacement of the tube but much more seriously, replacement of the complete charge of refrigerant plus in some instances, loss of refrigerated food products. In many larger installations, replacement of the refrigerant alone involves an expenditure of hundreds or even thousands of dollars.
One system that has alleviated the above described consequences of failure of the capillary tube of the control unit is outlined in U.S. Pat. No. 3,498,075. In the control unit disclosed therein, the pressure actuated switch is physically isolated from the flow control path of the refrigerant so that failure of the control unit or of the interconnection between the refrigerant and the pressure switch has no effect upon and does not result in the loss of the refrigerant. This isolation is provided by pressure actuated diaphragm motors or bellows located at opposite ends of a non-compressible capillary tube and interconnected by a pressure transmitting fluid enclosed within the tube. While this system serves admirably in overcoming the disadvantages noted above in control units in which the pressure actuated switch is connected directly to the refrigerant line, the system involves monitoring pressure changes in the refrigerant line by the pressure actuated switch in an indirect manner by means of an arrangement involving a secondary sensing device and a transmitting fluid, rather than directly by the pressure actuated switch as in the system it is intended to replace.